xen/blkback: Fix the inhibition to map pages when discarding sector ranges.

xen/blkback: Fix the inhibition to map pages when discarding sector ranges.

The 'operation' parameters are the ones provided to the bio layer while
the req->operation are the ones passed in between the backend and
frontend. We used the wrong 'operation' value to squash the
call to map pages when processing the discard operation resulting
in mapping the pages unnecessarily.

CC: Li Dongyang <lidongyang@novell.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>